1. Field of the Invention
The present invention relates to a zoom lens, and more particularly to a zoom lens suitable for an image pickup apparatus using a solid-state image pickup element or the like.
2. Description of the Related Art
Along with the functional sophistication of video cameras and digital cameras using solid-state image pickup elements, the optical systems used in those cameras are also called on to be further sophisticated in performance and reduced in size at the same time.
This kind of camera is required to have a lens system having a relatively long back focus because various optical members including a low pass filter and a color correction filter have to be arranged between the rear end of the lens system and the image pickup element. Furthermore, in a camera using a color pickup element, the lens system should preferably excel in telecentric characteristics on the image side in order to avoid color shading.
According to the prior art, there have been proposed various zoom lenses of the so-called short zoom type, having two lens units, negative and positive. In such a short zoom type optical system, the variation of magnification is achieved by moving the positive second lens unit, and the compensation for variation of the image point position by the variation of magnification is achieved by moving the negative first lens unit.
Further examples in pursuit of both higher performance and smaller size include attempts at correcting for aberrations by arranging a negative or positive third lens unit on the image side, described in the Japanese Patent Publication No. 7-3507 (corresponding to the U.S. Pat. No. 4,810,072) and the Japanese Patent Publication No. 6-40170 (corresponding to the U.S. Pat. No. 4,647,160). However, since these lens systems are mainly designed for use with 35 mm photographic films, they can be hardly considered compatible with the long back focus and superior telecentric characteristics required of optical systems using solid-state image pickup elements.
Zoom lens systems satisfying the back focus and telecentric requirements include the optical systems each having three lens units, negative, positive and positive, described in the Japanese Patent Laid-Open No. 63-135913 (corresponding to the U.S. Pat. No. 4,838,666) and the Japanese Patent Laid-Open No. 7-261083. The Japanese Patent Laid-Open No. 3-288113 (corresponding to the U.S. Pat. No. 5,270,863) also discloses an optical system having three negative-positive-positive lens units with the negative lens unit being fixed and the positive second lens and third lens units being moved to achieve the variation of magnification. These examples of the prior art, however, involve the drawbacks of a relatively large number of single lenses constituting each lens unit, a great overall length of the lens system and a high manufacturing cost.
In another example described in the Japanese Patent Laid-Open No. 7-261083, a convex lens is arranged as the closest lens of a negative first lens unit toward the object, and this involves the disadvantage that the outer diameter of the lenses is inevitably increased particularly when the angle of field is widened. Moreover in this example, as an object at a short distance is focused on by moving the negative first lens unit, this arrangement combines with the movement for zooming to complicate the mechanical structure.
In the U.S. Pat. No. 4,999,007 is disclosed a configuration of three negative-positive-positive lens units, wherein the first lens unit and the second lens unit are composed of one single lens each. However, as the overall length of the lens system at the wide angle end is relatively great and a wide gap between the first lens unit and the aperture-stop at the wide angle end results in a great incidence height of off-axis light rays, this configuration invites an increase in the diameter of the lens constituting the first lens unit with the adverse consequence of a large overall size of the lens system.
Moreover, since the first lens unit and the second lens unit consist of single lens, respectively, the correction of aberrations within the lens unit is insufficient. In particular, whereas variations in chromatic aberration of magnification in the process of zooming are more likely than elsewhere to occur in the first lens unit wherein the height of off-axis light rays from the optical axis varies more greatly, as the first lens unit consists of single concave lens, no correction is done within the lens unit, further inviting a problem that the chromatic aberration of magnification varies greatly in the whole system as well.
Furthermore, as a peculiar problem with expanding the angle of view at the wide angle end of zooming, there is a shortage of insufficient correction of distortion. Where use with a high pixel image pickup element of a relatively low sensitivity level is intended, an even greater aperture ratio is required.
In the U.S. Pat. No. 4,824,223, there is disclosed an optical system for use in projectors having a configuration of three negative-positive-positive lens units. As the first lens unit in this lens system consists of single negative lens, there is no correction of aberrations within the lens unit, and the variable magnification ratio is no more than 1.7 or so.
In view of these problems, the assignee of the present application disclosed in the Japanese Patent Laid-Open No. 2000-111798 (corresponding to the U.S. Pat. No. 6,308,011), an image pickup lens system having a configuration of three negative-positive-positive lens units. This image pickup lens system, while securing both a required lens-back space for inserting a filter or the like and the telecentric performance required for a solid-state image pickup element, is minimized in overall length while keeping a variable magnification ratio of no less than 2, resulting in a compact zoom lens.
In the zoom lens according to the Japanese Patent Laid-Open No. 2000-111798, the aperture stop, together with the second lens unit mainly accountable for the magnification varying action, moves toward the object side in the zooming from the wide angle end to the telephoto end. This results in a wide difference between the wide angle end and the telephoto end in the distance between the aperture stop and the image plane, and accordingly in greater susceptibility to variations of the exit pupil. There is a problem that, especially when the variable magnification ratio is raised, the exit pupil varies more because the moving distance of the second lens unit is increased.
The Japanese Patent Laid-Open No. 6-94996 discloses a configuration of three negative-positive-positive lens units, in which an aperture stop fixed during the process of zooming is arranged between the first lens unit and the second lens unit to keep exit pupil variations relatively small. However, because the aperture stop is fixed during the process of zooming, it is necessary to secure a longer distance between the aperture stop and the second lens unit than the moving distance of the second lens unit, and this gives rise to a problem that the overall optical length at the wide angle end is increased. There is a further problem that the lens diameter of the second lens unit increases because the second lens unit is relatively far from the aperture stop at the wide angle end.
The Japanese Patent Laid-Open No. 9-211326 also involves similar problems.
According to the Japanese Patent Laid-Open No. 62-200316, in a configuration of three negative-positive-positive lens units, an aperture stop moving toward the object side in zooming from the wide angle end to the telephoto end is arranged between the second lens unit and the third lens unit. This is a flare cut stop for effectively cutting flare light beams in the whole range of variable magnification. What limits the axial light rays are the aperture stop in the second lens unit, and this is unsuitable for a camera using a solid-state image pickup element because exit pupil variations arise in the process of zooming.
Configurations set forth in the Japanese Patent Laid-Open No. 2-118509 and the U.S. Pat. No. 4,810,072 also involve similar problems.
A zoom lens disclosed in the Japanese Patent Laid-Open No. 11-305125 (corresponding to the U.S. Pat. No. 6,154,322) has a configuration of two negative-positive lens units, with the aperture stop being arranged between the first lens unit and the second lens unit to be able to move independent of the second lens unit during the process of zooming. However, there also is a problem that, on account of the absence of a positive third lens unit, the exit pupil itself is not kept at a long enough distance from the image plane. There is a further problem that, because of the configuration having two lens units, the overall optical length is increased especially at the telephoto end by the movement of both the first lens unit and the second lens unit toward the object side in zooming from the wide angle end to the telephoto end.